1. Field of the Invention
The present invention relates to a code sequence generation method, modulation apparatus, modulation method, modulation program, demodulation apparatus, demodulation method, demodulation program and storage medium, and is preferably applied to an optical disc device, for example.
2. Description of the Related Art
An optical disc device is here to stay: the optical disc device records data on optical discs such as Compact Disc (CD), Digital Versatile Disc (DVD) and “Blu-ray Disc (Registered Trademark)”, and reads data from the optical disc to play back.
To prevent errors of readout or the like, the optical disc device converts data into a predetermined code sequence, and then records the codes on the optical disc.
The optical disc device records data in the following manner: the optical disc device converts, in accordance with a predetermined modulation method, data into code sequences which are suitable for recording on optical discs, and records the code sequences on the optical disc. The optical disc device reproduces data in the following manner: the optical disc device reads out code sequences from the optical disc, and demodulates, in accordance with a demodulation method corresponding to the above modulation method, the code sequences to reproduce the original data.
By the way, the code sequences are recorded on an optical disc (i.e. a storage medium) in the following manner: pits with discrete length are formed on the disc along tracks. With these optical discs, channels for recording or transferring the code sequences have a predetermined frequency spectrum.
The code sequences which have been generated in accordance with a predetermined rule will have a unique frequency spectrum.
Accordingly, the optical disc device brings the code sequence's frequency spectrum close to that of the optical disc (or the channel). That is to say, in the optical disc device, the code sequences with almost the same frequency spectrum as that of the optical disc is applied. This eliminates errors of readout of the code sequences from the optical discs and the like, and improves reproduction characteristics of data.
Therefore, Matched Spectral Null (MSN) codes are disclosed in the following documents: L. Fredrickson, “Improved Trellis-Coding for Partial-Response Channels”, IEEE TRANSACTIONS ONMAGNETICS, VOL 31, NO. 2, MARCH 1995; Jpn. Pat. Laid-open Publication No. H10-173536; and Jpn. Pat. Laid-open Publication No. H11-186917. In the case of MSN codes, Partial Response (PR) equalization process is performed while reproducing data, and a null point of the frequency spectrum of reproduced signals (which are after the PR equalization process is performed) and a null point of the frequency characteristics of the code sequences are matched.